A number of integrated circuit microprocessor chips have been implemented using advanced MOS LSI (metal oxide semiconductor large scale integrated circuit) manufacturing processes, including the P-channel and the faster, more sophisticated N-Channel technologies. The microprocessor chips are the heart of complete microcomputer systems which additionally typically include a number of power supplies, external RAM (random access memory) devices, ROM (read only memory) devices and a variety of buffer interface and peripheral control devices. A relatively small number of semiconductor chips are required to implement a complete microcomputer system. A simple but complete microcomputer using such microprocessor packages may be implemented on a small printed circuit card, requiring as few as four or five integrated circuit packages. Some control functions may be implemented with even fewer integrated circuit packages. Thereby, the power of computer data processing is made available at very low cost for use in a wide variety of industrial and communications equipment, such as in process and manufacturing control systems, computer peripheral and terminal hardware, parameter-control systems of all types from microcomputers in the automobile to the control systems for traffic and anywhere else that random logic computer control needs optimizing. The savings in design time and cost for the microcomputer systems designer is greatly reduced by the availability of such MOS LSI microprocessor and memory and peripheral units. Printed circuit board layouts are simplified. The complex interconnections required for large numbers of conventional integrated circuits are replaced by ROMs. The only interconnect wiring on printed circuit cards runs between the various address and data buses and input/output devices. The cost savings are not limited to direct component costs, but extends also to other related system hardware costs. Connectors can be decreased in number, cabling can be simplified, the card cage can be reduced in size, and so on. Associated indirect costs are also reduced, since assembly takes less time, documentation is simpler, and maintenance is easier. Equally important to cost savings and hardware systems is the ability of system engineers to build a proposed design quickly. No hard wire logic need be simulated, optimized or breadboarded. The logic design portion of the design cycle now becomes manipulation of the functional building blocks mentioned above, where the control sequence takes the form of writing a software program into an external ROM. Breadboarding consists merely of interconnecting a few LSI packages. Design changes are greatly simplified. The use of simulators allows verification of the design even before it is committed to hardware. Overall, as much as 90 percent may be cut from the design time of a microcomputer using the microprocessor and related chips. However, different microprocessors have different capabilities which offer different advantages to users having varying requirements. For instance, one microprocessor chip may be clearly advantageous to one user and a different microprocessing unit may be useful to a different user with different types of program requirements. The effectiveness of the particular microcomputer systems can usually be measured by its execution speed and by its external memory storage requirements for a representative class of practical bench mark programs. It is also clear that a microprocessor unit having shorter machine cycles requiring fewer machine cycles to execute a particular set of instructions will be generally advantageous to any user.
Ease of controlling the input/output data flow may also offer the user advantages in using a particular microprocessor unit. Now families of support circuits coupleable directly to a bidirectional data bus, which characterizes most of the known microprocessor chips, and the new microprocessor units (MPU's) are far more powerful, generally, than the earliest first generation MOS LSI microprocessor chips in that the instruction sets for the newer devices are much larger and more powerful (the most advanced microprocessor chips include about 70 instructions) and the instruction execution times are about on order of magnitude faster. Yet, further improvements in overall system performance may be based on improvements in chip architecture of the microprocessor chips and new systems architectures encompassing the MPU chips and associated peripheral circuit families.